Method and device for equalizing resistance of heating element of thermal head of thermal printer

Abstract

When setting up the thermal head, respective resistances of the heating elements of the thermal head are measured, and a difference between the resistance value of each heating element and the smallest resistance value is detected. A predetermined amount of resistance trimming energy for lowering the resistance of the heating element by a predetermined constant amount is applied to each heating element for a number of times which depends on the detected difference in the resistance of the heating element from the smallest resistance, to trim the resistance of the heating element down to the smallest resistance.

What is claimed is: 
     
       1. A method of equalizing resistances of heating elements of a thermal head which are arranged in an array, said method comprising the steps of: (A) measuring respective resistance values of said heating elements;   (B) extracting and determining a smallest resistance value from among said resistance values of said heating elements;   (C) detecting a difference between said smallest resistance value and a larger value of said resistance values; and   (D) applying a trimming energy of an amount variable in accordance with said difference to a first one of said heating elements which has said larger value so as to trim a resistance for said first one of said heating elements.   
     
     
       2. The method as recited in claim 1, wherein said trimming energy is obtained by applying a first voltage, which is higher than a print voltage for driving said heating elements in printing and is variable depending on said difference, for a constant time, or by applying a second voltage which is constant and higher than said print voltage for a time variable depending on said difference. 
     
     
       3. The method as recited in claim 2, further comprising measuring the resistance for said first one of said heating elements again at a mid time of application of said trimming energy, and thereafter, repeating said steps (C) and (D) on the basis of the resistance measured at said mid time for said first one of said heating elements. 
     
     
       4. A method of equalizing resistances of heating elements of a thermal head which are arranged in an array, said method comprising the steps of: (A) measuring respective resistance values of said heating elements;   (B) extracting and determining a smallest resistance value from among said resistance values of said heating elements;   (C) detecting a difference between said smallest resistance value and a larger value of said resistance values;   (D) dividing said difference by a predetermined constant resistance reduction value to determine a number of times a constant amount of a trimming energy should be applied to a first one of said heating elements which has said larger value, wherein said predetermined constant resistance reduction value represents an amount of resistance reduction obtained by applying said constant amount of said trimming energy; and   (E) applying said constant amount of said trimming energy to said first one of said heating elements for said number of times determined in said step (D), thereby heating said first one of said heating elements and reducing a resistance of said first one of said heating elements to be within a predetermined amount of said smallest resistance value.   
     
     
       5. The method as recited in claim 4, wherein said constant amount of said trimming energy represents an electric energy obtained by applying a constant voltage for a constant time, said constant voltage being higher than a print voltage used for driving said heating elements in printing. 
     
     
       6. A method of equalizing resistances of heating elements of a thermal head which are arranged in an array, said method comprising the steps of: (A) measuring respective resistance values of said heating elements;   (B) extracting and determining a smallest resistance value from among said resistance values of said heating elements;   (C) detecting a difference between said smallest resistance value and a larger value of said resistance values;   (D) calculating a number of application repetitions for a constant amount of a trimming energy to a first one of said heating elements which has said larger value, according to the following equation:   X=(ΔRi-M)/ΔR.sub.TRIM ;       in which X is said number of application repetitions of said constant amount of said trimming energy, said constant amount of said trimming energy reducing said resistance values of said heating elements by a constant resistance reduction value, .sup.Δ  Ri is said difference between said larger value and said smallest resistance value, ΔR TRIM  is said constant resistance reduction value, and M is a predetermined value which is a predetermined number of times greater than said constant resistance reduction value;   (E) applying said constant amount of said trimming energy to said first one of said heating elements for a first predetermined number of times;   (F) measuring a reduced resistance value of said first one of said heating elements;   (G) detecting a new difference between said reduced resistance value and said smallest resistance value; and   (H) applying said constant amount of said trimming energy again to said first one of said heating elements for a second predetermined number of times, wherein said second predetermined number of times is determined according to the following equation:   Y=ΔRi.sub.AFTRIM /ΔR.sub.TRIM       in which Y is said second predetermined number of times, and ΔRi AFTRIM  is said new difference between said reduced resistance value and said smallest resistance value.   
     
     
       7. The method as recited in claim 6, wherein said resistance measuring step (A) comprises the steps of: (A)(1) charging a capacitor up to a first voltage level with said capacitor being connected in parallel to said heating elements;   (A)(2) discharging said capacitor through a reference resistor of a known resistance which is connected in parallel to said heating elements;   (A)(3) measuring a first discharge time required to discharge said capacitor through said reference resistor from said first voltage level to a second voltage level;   (A)(4) turning one of said heating elements ON, so as to discharge said capacitor through said one of said heating elements whose resistance is to be measured;   (A)(5) measuring a second discharge time required to discharge said capacitor through said one of said heating elements from said first voltage level to said second voltage level;   (A)(6) calculating a resistance value of said one of said heating elements on the basis of said measured said first and second discharge times; and   (A)(7) storing said resistance value in a memory.   
     
     
       8. An apparatus for equalizing resistances of heating elements of a thermal head which are arranged in an array, said apparatus comprising: memory means for storing unit trimming data including a constant resistance reduction value and a constant amount of a trimming energy for reducing resistances of heating elements by said constant resistance reduction value;   measuring means for measuring respective resistance values of said heating elements;   extracting means, receiving said resistance values from said measuring means, for extracting and determining a smallest resistance value from among said resistance values of said heating elements;   detecting means, receiving said smallest resistance value from said extracting means, for detecting a difference between said smallest resistance value and a larger value of said resistance values;   determining means, receiving said difference from said detecting means and said unit trimming data from said memory means, for determining, with reference to said unit trimming data and depending on said difference, a number of times said constant amount of said trimming energy should be applied to a first one of said heating elements which has said larger value; and   applying means, receiving said number of times from said determining means, for applying said constant amount of said trimming energy to said first one of said heating elements for said number of times determined by said determining means.   
     
     
       9. The apparatus as recited in claim 8, wherein said applying means comprises: power supply means for supplying said trimming energy to said heating elements;   an array of switches each for connecting and disconnecting one of said heating elements to said power supply means; and   controlling means, receiving said number of times from said determining means, for controlling said array of switches to selectively supply said heating elements with said constant amount of said trimming energy.   
     
     
       10. The apparatus as recited in claim 9, wherein said constant amount of said trimming energy represent an electric energy obtained by applying a constant voltage for a constant time, said constant voltage being higher than a print voltage used for driving said heating elements in printing. 
     
     
       11. The apparatus as recited in claim 8, wherein said power supply means is switched to output either said voltage for said trimming energy or said print voltage. 
     
     
       12. A thermal printer having a thermal head whose heating elements are arranged in an array, said thermal printer comprising: memory means for storing unit trimming data including a constant resistance reduction value and a constant amount of a trimming energy for reducing resistances of heating elements by said constant resistance reduction value;   measuring means for measuring respective resistance values of said heating elements;   extracting means, receiving said resistance values from said measuring means, for extracting and determining a smallest resistance value from among said resistance values of said heating elements;   detecting means, receiving said smallest resistance value from said extracting means, for detecting a difference between said smallest resistance value and a larger value of said resistance values;   determining means, receiving said difference from said detecting means and said unit trimming data from said memory means, for determining, with reference to said unit trimming data and depending on said difference, a number of times said constant amount of said trimming energy should be applied to a first one of said heating elements which has said larger value;   power supply means for supplying said trimming energy to said heating elements;   an array of switches each for connecting and disconnecting each of said heating elements to said power supply means; and   controlling means, receiving said number of times from said determining means, for controlling said array of switches to selectively supply said heating elements with said constant amount of said trimming energy for said number of times that is determined by said determining means.   
     
     
       13. The thermal printer as recited in claim 12, wherein said constant amount of said trimming energy represents an electric energy obtained by applying a constant voltage for a constant time, said constant voltage being higher than a print voltage used for driving said heating elements in printing. 
     
     
       14. The thermal printer as recited in claim 13, wherein said power supply means is switched to output either said voltage for said trimming energy or said print voltage. 
     
     
       15. A method of equalizing resistances of heating elements of a thermal head of a thermal printer which are arranged in an array, said method comprising the steps of: (A) measuring respective resistance values of said heating elements;   (B) extracting and determining a largest resistance value from among said resistance values of said heating elements;   (C) detecting a difference between said largest resistance value and a smaller value of said resistance values;   (D) determining a resistance trimming value depending on said difference with respect to a resistance increase amount which is experimentally detected by applying a predetermined amount of an electric energy to a test heating element having a construction equivalent to a construction of said heating elements; and   (E) applying a variable amount of said electric energy in accordance with said resistance trimming value, to a first one of said heating elements which has said smaller value, so as to heat said first one of said heating elements and increase a resistance of said first one of said heating elements to be within a predetermined amount of said largest resistance value.